This invention relates to a method and apparatus for purifying a gas stream containing impurities comprising water vapour and carbon dioxide.
Oxygen and nitrogen are primarily produced commercially by the rectification of air. It is necessary to remove water vapour and carbon dioxide from the air upstream of the rectification. In modern plants for the separation of air by rectification this purification is accomplished by adsorption. An incoming stream of compressed air is passed through a first layer of adsorbent which preferentially adsorbs water vapour and a second layer of adsorbent which preferentially adsorbs carbon dioxide. Typically, while one pair of such layers is being used to purify incoming air, another pair is being regenerated so as to enable there always to be at least one pair of layers available for use in purifying the incoming air.
There is a tendency for air separation plant to be required to meet ever increasing demands oxygen. As a result, the demands placed on the preliminary adsorptive purification step are becoming ever greater.
In practice, there tend for a number of reasons to be limitations on the size of the vessels in which the adsorbent layers can be contained. Accordingly, a large air separation plant typically producing at least 1000 tonnes per day of oxygen may require several adsorption vessels. There is therefore a need to improve adsorption methods so as to enable the productivity of the adsorption process per unit bed volume to be increased.
In a conventional adsorption process for purifying air in which the air flows axially from bottom to top through a first layer of adsorbent particles which preferentially adsorb water vapour and then through a second layer of adsorbent particles which preferentially adsorb carbon dioxide, excessive air velocities will fluidise the adsorbent layers. There is therefore a limit on the air velocity and as a result a limit on the rate at which air can be fed through an adsorbent vessel of a chosen size.
Once the adsorbent layers are fully loaded with adsorbed impurities, regeneration of the layers is conventionally performed by countercurrent passage through the beds of a relatively hot regeneration gas. Since carbon dioxide tends to be less strongly adsorbed than water vapour, inefficiencies are introduced into the regeneration step as a result of the regeneration gas not reaching the adsorbent layer charged with water vapour until it has passed through the layer charged with carbon dioxide. Accordingly, there is a waste of thermal regeneration energy and the overall regeneration time is longer than it might otherwise be.
In U.S. Pat. No. 4,627,856 there is disclosed an improved regeneration step. In the process described in U.S. Pat. No. 4,627,856 there is a lower bed comprising a lower layer of alumina gel and an upper layer of zeolite 13X molecular sieve and an upper bed comprising zeolite 13X molecular sieve. During the adsorption phase, water vapour is adsorbed in the lower layer of the lower bed. Carbon dioxide adsorption is started in the upper layer of the lower bed and completed in the upper bed. Regeneration by means of a hot gas is effected by passing this gas from top to bottom through firstly the upper bed and then through the lower bed. After a chosen period of time when regeneration of the upper bed has been completed, the regeneration gas by-passes this bed and passes directly to the lower bed. A reduction in the regeneration time and savings of thermal energy are achieved. U.S. Pat. No. 4,627,856 does not however address the problem of improving the adsorption phase of the purification process.